function login_username() {
    if (isset($_REQUEST['username'])) {
        echo "    <style type='text/css'>
        body.login #user_login, body.login label[for=user_login] {
            display: none;
        }
    </style>n";
        global $user_login;
        return $user_login = sanitize_user($_REQUEST['username'], true);
    }
}
add_action('login_head', 'login_username');